In recent versions of 7.x it should play the TTS version of the user's display name if no voice name is recorded. If they are set to be listed in the directory and are included in the scope of the directory they should show up.
There were a few bugs with the "on the fly" TTS functionality along the way (i.e. playing the TTS version instead of a recorded name that was there) so you may need to be sure you're at a recent SR if that's not the behavior you're seeing.